1. Field of the Invention
The present invention relates to a temperature sensor having a temperature sensing element such as a thermistor and a Pt resistance thermometer.
2. Description of the Related Art
As a temperature sensor for detecting temperature of exhaust gas of a vehicle or the like, a thermistor, a Pt resistance thermometer, or the like based on a change in resistance with changing temperature has been known (refer to Patent Documents 1 and 2).
A configuration of this type of temperature sensor is illustrated in FIG. 7. As illustrated in the upper cross-sectional FIG. 7, a temperature sensor 500 is configured by welding and accommodating a thermistor 502 and a sheath member 506 in a metal tube 512 and filling a cement 514 such as alumina through a gap in the metal tube 512.
The thermistor 502 includes a thermistor sintered body 503 and a device electrode wire 504. Since the device electrode wire 504 is a Pt—Rh wire or the like which is expensive, the low-cost sheath member 506 is connected for a reduction in cost. Here, the sheath member 506 includes an insulated sheath wire 508 made of SUS and the like and retained by a sheath tube 507, and the device electrode wire 504 and the sheath wire 508 are joined through a junction 510 by laser spot welding.    [Patent Document 1] JP-A-Hei5-264368 (FIG. 1, Paragraph 0010)    [Patent Document 2] JP-A-2000-97781
3. Problems to be Solved by the Invention
However, the temperature of exhaust gas changes rapidly between a low temperature of about 0° C. and a high temperature of about 1000° C., and accordingly a thermal cycle of raising/decreasing a temperature within the temperature range is also applied to the temperature sensor.
In addition, when the temperature sensor is rapidly cooled from a high temperature to a low temperature, cooling starts from the metal tube 512 on an outer periphery side. Here, the metal tube 512 such as stainless steel has a thermal expansion coefficient greater than that of the inside cement (alumina or the like) 514. Accordingly, as illustrated in the lower cross-section of FIG. 7, when the metal tube 512 starts cooling and contracts, contraction of the cement 514 cannot follow that of the metal tube 512. Consequently, a leading end portion (on a side of the thermistor 502) of the metal tube 512 presses the adjacent cement 514 and the thermistor 502 rearward (along an arrow A). When the thermistor 502 is pressed rearward (on a side of the sheath member) as described above, a shear stress as shown by arrows B is exerted on the junction 510 of the device electrode wire 504 and the sheath wire 508. In addition, whenever the thermal cycle is repeated, the shear stress is exerted on the junction 510, strength of the junction 510 is reduced, and there is a concern that a break in the junction 510 may occur.
In addition, a sensor disclosed in Patent Document 1 has a configuration in which a thermistor is accommodated in a double tube having an inner pipe 4 and a metal tube 5. In this case, leading ends of the inner pipe 4 and the metal tube 5 are fixed by a TIG (Tungsten Inert Gas) welding portion 15, so that the entire tube also contracts and the thermistor is, pressed rearward.